Custom media effect in Win10 Universal App - win-universal-app

Question

I'm developing an Universal Windows App that uses the camera and I need to create a custom effect for the camera. The app is written in C# and the effect is a C++ DLL that uses Media Foundation and an IDL to define a custom WinRT component.
I already successfully do this several times on Windows 8.1 with the following trick documented here, or here.
I now want to port this on Windows 10 UWP. I'm aware there is a known issue on Windows 10 which make the compiler fail with the error "Failed to load a dependency file. Windows.winmd" (see here). But even with that fix, I'm not able to successfully build the component.
Here is the IDL:
import "Windows.Media.idl";
#include <sdkddkver.h>
namespace MyEffect
{
[version(NTDDI_WIN10), activatable(NTDDI_WIN10)]
runtimeclass MyEffect
{
[default] interface Windows.Media.IMediaExtension;
}
}
The MIDL command line (formatted for better readability):
/env win32 /h "MyEffect_h.h" /W1 /char signed /enum_class
/tlb "$(OutDir)\MyEffect.tlb" /ns_prefix
/metadata_dir "$(WindowsSDK_MetadataFoundationPath)"
/nologo /winrt
/winmd "$(OutDir)\MyEffect.winmd"
This command succeeds. Then I add the following custom build step:
mdmerge -metadata_dir “<metadata_dir>”
-i “$(OutDir).”
-o “$(OutDir)merged”
-partial
-v
and this command fails always with different errors according to the following :
If I put $(WindowsSDK_MetadataFoundationPath), which resolves to c:\Program Files (x86)\Windows Kits\10\References\windows.foundation.foundationcontract\1.0.0.0, or $(WindowsSDK_MetadataPath), which resolves to c:\Program Files (x86)\Windows Kits\10\References it returns the error:
MDMERGE : error MDM2002: The type "Windows.Media.IMediaExtension" defined in file <...>\MyEffect.winmd was referenced by type MyEffect.MyEffect but could not be found.
If I put $(TargetPlatformSdkMetadataLocation), which resolves to c:\Program Files (x86)\Windows Kits\10\References\CommonConfiguration\Neutral, it returns the error:
MDMERGE : error MDM2012: Error 3 has occured while enumerating files in C:\Program Files (x86)\Windows Kits\10\References\CommonConfiguration\Neutral\*.winmd.
Does anyone figured out how to compile a custom Windows Media effect for UWP ?

Answer 1

Setting -metadata_dir to "C:\Program Files (x86)\Windows Kits\10\UnionMetadata" worked for me.